SKP Scanning Kelvin Probe Microscopy

The Kelvin Probe experiment uses a nondestructive method to determine the relative work function difference between the probe and the sample. Work function describes the energy required to liberate an electron from the surface of a conductor; electrochemists often interpret this as the difference from an electrode’s Fermi Level, average energy of electrons, and that of vacuum.

A metal microprobe is positioned close to the surface of the sample (on the order of 100-microns). If the microprobe and sample are of different metals, there is an energy difference between their electrons. The microprobe is then electrically shorted to the sample, through internal electronics of the system. As a consequence, one metal forms a positive charge on its surface and the other metal forms a negative charge on its surface. The probe and sample are separated by a dielectric (air), so a capacitor is formed. The probe is then vibrated and "backing potential" or "nulling potential" is then applied sufficient to minimize this capacitance. At the applied voltage that causes the capacitance to go to zero, the original state is achieved. This value is recorded and charted.

Experiments are typically performed in ambient gaseous conditions, but several published examples use humidified environments. The underlying conducting sample can have an organic coating or paint applied.

This relative work function can also be correlated to an Ecorr value.

Our SKP is also capable of functioning in Topography Mode. Without changing connections or probe, a reference voltage is applied to the sample. This reference voltage makes the surface of the sample uniform. Change in capacitance is then from a changing plate separation (via equation of a capacitor).

This information can be used in 2 ways:
Position the probe a known distance from the sample, using a Calibration Coefficient.

Map topography for further use in Constant-Distance Mode SKP. This is particularly useful in studying welds or other samples of complex topography.

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VersaSCAN Electrochemical Scanning Microscopy Techniques Overview

Experiments
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Line Scan

Scanning Kelvin Probe (SKP) measures the relative work function difference between the probe and sample. The probe is scanned in the XY plane above the sample, generating a map of the response. The probe can be maintained at a constant-height or a constant-distance (if a topographic background scan is acquired first). Step-scan data acquisition is recommended for oversampling and averaging of the data, also this gives the logic time to determine the appropriate nulling voltage.

Area Scan

Capacitive Height Measurement (CHM) applies a voltage to the sample. A calibration constant is determined to relate the measured probe response to a distance…over a relatively small change in distance. The same SKP probe is scanned across the XY plane above the sample and the response is reported as a probe-to-sample distance (mountains are low-values and valleys are large-values). These data could then be used as a background to run an additional SKP experiment in constant-distance mode.

CTM Area Scan

CHM applies a voltage to the sample. A calibration constant is determined to relate the measured probe response to a distance…over a relatively small change in distance. The same SKP probe is scanned across the XY plane above the sample and the response is reported as a probe-to-sample distance (mountains are low-values and valleys are large-values). These data could then be used as a background to run an additional SKP experiment in constant-distance mode.